Mattresses and other types of cushions have for decades been constructed to be “double-sided” or in other words symmetrical in cross-section, wherein the configuration and arrangement of materials and components is identical on each side. Double-sided symmetrical construction enables flipping of the cushion or mattress to obtain the same support characteristics on a fresh uncompressed side. It was long held that this was necessary to allow compressed layers of padding, particularly natural materials such as cotton batting or fowl feathers, to decompress while the opposite side was used as the support side. But with the advent of improved materials for the padding layers, including foam materials with excellent resilience which promptly return to an uncompressed or substantially uncompressed state, the padded support side does not require a prolonged recovery period as was provided by flipping to an opposite side, and in fact recovers quickly when decompressed and can maintain this performance for the life of the product. This has led to the recent development of “one-sided” mattresses, designed and constructed to have only one support permanent support side or surface, with an opposite side designed for permanent support by and contact with the top side of a box spring or foundation. One-sided or “no-flip” mattresses are thus designed to concentrate essentially all of the support and comfort features at or near the single support side, with the opposite or bottom side serving only as a platform for support by a foundation. The amount and quality of padding and other filling materials at or near the support side is therefore dramatically greater than at the opposite bottom side.
Mattresses, seating and other flexible support structures have conventionally been constructed with multiple interconnected spring components, such as steel wire springs of various configurations, which are covered with the described layers of padding and upholstery at the support side or sides of the innerspring. In double-sided mattresses with the described symmetrical layers of material on each opposing side, the internal spring components are symmetrical in both vertical and horizontal dimensions, so that they provide the same resistance forces at each end and collectively to each supporting side of the mattress. Symmetrical spring designs are also preferred for and ubiquitous in automated manufacturing by wire-forming machines which form a helical coil spring body and then form the ends of the spring with impact dies. The symmetry of a spring component about a horizontal plane means that an upper portion of the coil (on a top side of the plane) is similar in size, shape and relative position of corresponding parts same as a bottom portion of the coil (on a bottom side of the plane). The term “symmetric” is defined as having similarity in size, shape and relative position of corresponding parts. Webster's Revised Unabridged Dictionary, 1996.
In flexible support structures with a fixed orientation, such as a mattress foundation, “box spring”, or sofa, springs may be mounted at one end to a framework such as a wooden frame, with the opposite ends defining a flexible support surface over which padding is placed. Springs used in this type of application may have a mounting end which is configured or shaped differently than an opposite support end, with the body of the coil transitioning from the mounting end to the support end. Coiled wire form type springs typically have a helical body which extends between ends of the coil. The helix which forms the coil body is at a fixed helical angle or pitch, primarily due to the wire forming machinery which uses a fixed gearing or cam to form the wire into a coiled helix. This gives the coil spring a fixed spring rate throughout its length and range of compression, so that the coil has a constant support characteristic or feel when compressed. Also, it is significant that in coil springs of this type the amount of material used to form the spring is the same throughout the length of the coil, even though the coil may only be compressed in a top quarter or third of its total length. With the majority of compression of a coil spring taking place only in the top quarter or third of the coil height, it is not necessary for the bottom three quarters or two thirds of the coil spring to be identically configured for good spring performance. The springs are symmetric only because they are installed in a symmetrical two-sided mattress where they must provide the same reflexive support to each side of the mattress when oriented up as the support side.